1. Field of the Invention
This invention relates to a system for detecting misfire for internal combustion engines that detects engine misfire based on ionization current occurring upon combustion of an air-fuel mixture, particularly to such a misfire detection system for internal combustion engines that prevents false detection of misfire owing to fluctuation in the discharge period of, or noise produced by, an ignition coil.
2. Description of the Related Art
In a gasoline or other spark-ignition internal combustion engine, a high voltage generated by an ignition coil is applied through an ignition distributor or the like to ignition plugs installed in the individual cylinders. The spark discharge that the high voltages produces across the gap between the ignition plug electrodes ignites the air-fuel mixture, causing combustion. However, when certain causes are present during the engine ignition/combustion stroke, the combustion of the air-fuel mixture does not proceed normally, i.e., misfire occurs.
Causes of misfire fall in two classes, those attributable to the fuel system and those attributable to the ignition system. Misfire attributable to the fuel system is the result of either excessively lean or excessively rich air-fuel mixture. In this case, a spark discharge is produced across the gap of the ignition plug but the air-fuel mixture does not ignite. Misfire attributable to the ignition system is the result of ignition plug smoldering caused by adherence of unburned fuel to the ignition plug or of a problem in the ignition circuit that prevents normal spark discharge (mis-sparking).
When the air-fuel mixture burns normally, the combustion is accompanied by ionization of the air-fuel mixture (more precisely the combustion gas produced by normal burning of the air-fuel mixture) that gives rise to ionization current. When misfire occurs and the air-fuel mixture does not burn, the air-fuel mixture does not ionize and no ionization current arises.
FIG. 10 shows the ionization current waveforms during misfire and normal combustion, in which the discharge is produced by a high voltage of negative polarity. As shown, the ionization current waveform during normal combustion, i.e., when ions are produced, spikes instantaneously in the minus direction just after the discharge across the ignition plug electrodes (as seen at A in the drawing), thereafter continues to flow in proportion to the volume or number of ions produced, and eventually returns to a given level. During misfire, i.e., when ions are not produced, the waveform spikes instantaneously in the minus direction just after the end of discharge (as seen at Axe2x80x2 in the drawing) and then immediately returns to the given level.
A widely used method of detecting engine misfire has therefore been to detect the ionization current (current waveform) occurring during the combustion stroke using the ignition plug, more exactly the electrodes thereof, as a probe for detecting ionization current, and comparing the detected value with a prescribed value, as taught by, for example, Japanese Laid-open Patent Application No. Hei 5(1993)-99956.
When the ignition command current pulse to the ignition coil is turned ON (i.e., current is supplied), the current waveform spikes in the minus direction during both misfire and normal combustion (as seen at B and Bxe2x80x2 in the drawing). These spikes appearing at A (Axe2x80x2) and B (Bxe2x80x2) are caused by inductive noise produced by the electromagnetic induction of the ignition coil and some measure is needed to keep them from affecting misfire detection (discrimination).
In this regard, the aforesaid prior art reference (No. Hei 5(1993)-99956) teaches that the adverse effect of inductive noise can be eliminated to detect only the actual ionization current waveform and achieve accurate misfire detection (discrimination) by not performing current waveform detection (by masking) for a prescribed period after the ignition command current pulse to the ignition coil is turned ON and current is supplied (specifically, during the time from the rise of the current waveform indicated at B to the fall of the current waveform indicated at A).
This known technique cannot, however, totally eliminate the influence of inductive noise when the ignition coil discharge period fluctuates, such as shown by the alternate long and short dashed lines in FIG. 11. This is because the method taught by this prior art reference conducts masking over a prescribed period following the time point of turn-on of the ignition command current pulse to the ignition coil (the charging start point) as the start of the masking period. When the discharge period of the ignition coil is prolonged, therefore, an inductive noise-induced rise in the current waveform occurring after the masking period has passed (as indicated at C in the drawing) is erroneously detected and affects the detection (discrimination) of misfire.
Moreover, since current is constantly detected at times other than during the masking period, currents produced by various kinds of noise (such as indicated at D in the drawing) are also detected and have a similar adverse effect on misfire detection (discrimination). A situation may arise, therefore, in which the current waveforms indicated at C and D in the drawing are erroneously discriminated as the result of ionization current, even though a misfire has occurred and no ionization current is actually present.
The present invention was accomplished to overcome the foregoing problems and has as an object to provide a misfire detection system for internal combustion engines that is unaffected by fluctuation in ignition coil discharge period, various kinds of noise and the like, and can therefore ensure accurate misfire detection by preventing erroneous detection (erroneous discrimination) owing to such causes.
For overcoming the foregoing problem, this invention provides a system for detecting misfire for an internal combustion engine having an ignition plug, installed to face into a combustion chamber of a cylinder of the engine and connected to an ignition coil, which produces spark discharge when supplied with discharge current from the ignition coil to ignite air-fuel mixture in the combustion chamber; including: a current detection circuit which detects ionization current, that flows following the discharge current, during a period; a misfire detector which detects occurrence of misfire of the engine based on the detected current; and a processing delay circuit which inputs at least one of the discharge current and the ionization current and based on the inputted current, delays starting of the period by a time point which is not earlier than termination of the discharge current.